Alternatives
Considering the fact that her very existence is an alternative to gem norm, Garnet is in no place to judge other alternative relationships. To be honest, she really doesn’t understand what Rose Quartz sees in humans, but she doesn’t have to. As long as it works for Rose, and Rose doesn’t slack on her duties as a Crystal Gem, then there’s nothing Garnet can say.
Besides, humans have such short lives. Rose’s romances never last long, and to complain about something so temporary is just not dignified.
Seeing Greg Universe so inspired by fusion kind of gets to her, though. He understood in one instance exactly how wonderful and intimate it is; in fact, he spent three hours of his short life setting up a dancefloor on the beach, just so he can potentially experience it himself. For the last hour, he has been trying to dance like Pearl because he thinks that’s the only way to fuse with Rose. In all of her years, Garnet has never seen a human work so hard to understand a gem. All of that effort makes Greg better than the others, in Garnet’s book.
Does she think that this will work in the traditional way? No. However, she likes that he’s trying.
Maybe it’s a little impulsive to send Amethyst bolting after some driftwood so she can tell him how fusion is different for everyone and he just needs to relax and be himself in order for it to work. In fact, it absolutely is impulsive, but Greg really seems to take it to heart, and Garnet can only appreciate his implicit understanding.
Besides, by the end of the night it seems to her like Rose is on her way to actually making something beautiful with this one.